The field of the invention relates generally to a coal-to-liquids plant with power coproduction, and more specifically, to a method and system of power coproduction using byproduct methane and purge gas from a Fischer-Tropsch coal-to-liquids plant.
At least some known Fischer-Tropsch (FT) based coal-to-liquids plants produce methane (C1), ethane and ethylene (C2) as byproducts which are purged as a tail gas stream. In addition to C1 and C2 hydrocarbons, the tail gas contains a significant amount of unreacted syngas and carbon dioxide (CO2). FT plants are generally used commercially for gas to liquids (GTL) and are configured to recycle the tail gas back to a gasifier, or burn the tail gas for steam and power production. This is economically attractive for GTL since C1 and C2 is the fresh feed to the gasifier and steam and power facilities.
However, for a coal-to-liquids plant, the C1 may be 10-20 times more valuable than the coal on a heating value ($/BTU) basis. Thus it is not economical to recycle C1 to a coal fed gasifier. If a pipeline infrastructure is not available to transport the C1 to a point of more valuable use, then preferential conversion of the C1 to power is desirable. The relatively low heating value of the tail gas and variation in the CO2 content complicate combustion turbine power generation which requires a relatively constant heating value (Wobbe index) fuel.
Coal gasification produces a syngas that contains significantly more nitrogen than syngas from a GTL plant. The additional nitrogen requires more purge syngas from the FT synthesis loop to prevent nitrogen buildup. Coal gasification also produces significantly more waste heat and HP steam reducing the fuel gas demand.
The combination of no gasifier recycle, increased purge syngas, and decreased fuel gas demand causes a fuel gas imbalance.